Method for forming conductive film at room temperature

ABSTRACT

A method for forming a conductive film at room temperature is provided and includes steps of: adding AgNO 3  into a first solution of dodecanoic acid; dropping n-butylamine and a diluted aqueous solution of a reducing agent into the first solution in turn, so as to initially obtain the dodecanoate-protected silver nanoparticles as a capping ligand; then using cyclohexane as a solvent to apply the silver nanoparticles onto a surface of a substrate to form a patterned film of silver nanoparticles; and finally immersing the substrate into a high concentrated aqueous solution of a reducing agent to chemically reduce the patterned film of silver nanoparticles into a conductive silver film. Thus, a patterned film or circuit can be conveniently and rapidly formed, and the silver nanoparticles can be applied to flexible substrates with low material cost and temperature sensitivity.

FIELD OF THE INVENTION

The present invention relates to a method for forming a conductive filmat room temperature, and more particularly to a method using a reducingagent to chemically reduce a film of dodecanoate-protected silvernanoparticles pre-coated on the flexible substrate into a conductivesilver film at room temperature.

BACKGROUND OF THE INVENTION

Low-cost liquid direct printing technology can be applied to a varietyof patterned and deposition processes, such as processes of integratedcircuits (IC), glass substrate circuits of large-size liquid crystaldisplay (LCD), surface circuits of LED wafer, repair of IC opencircuits, electronic tags or radio frequency identification (RFID), etc.Traditionally, plating and etching to form a conductive patternedcircuit are generally done by lithography processes. However, due to themany complex steps required to construct a layer of circuit, it isrelatively time-consuming and the manufacturing cost is higher.Therefore, for the related industry, it is necessary to simplify theprocess and reduce the manufacturing cost of the direct printingtechniques, wherein the technique is to quickly convert the metalnanoparticles into the low-resistance metal conductor to form aconductive circuit after removing the solvent and heat treatment.

Nowadays, one of the direct printing technologies is to firstly mixmetal nanoparticles and a solvent as a printing ink, and then spray theprinting ink onto a substrate to form a conductive circuit by an inkjetmethod, wherein the metal nanoparticles is preferably selected fromsilver. Silver is more suitable than gold for being used in this art,because silver is the metal having the highest conductivity among allthe metals. And, gold has excessive material cost which affects itsapplicability in the field of electronics.

For example, the inventors of the present patent previously disclosed apaper named “One-step synthesis of uniform silver nanoparticles cappedby saturated decanoate: direct spray printing ink to form metallicsilver films” published in the journal, Physical Chemistry ChemicalPhysics (2009, vol. 11, p. 6269-6275), on May 27, 2009, wherein itdisclosed that the decanoate-protected silver nanoparticles(C₉H₁₉COO₂—Ag) is used as a printing ink. However, the stability of theoperation of the decanoate-protected silver nanoparticles at roomtemperature is only maintained for one day because of desorption by thecarbon dioxide and other decomposition fragments. Therefore, there is aneed to change other protective agents to stabilize the produced silvernanoparticles, or that will significantly affect the application of thesilver nanoparticles in the direct printing technology. Furthermore, thedecanoate-protected silver nanoparticles described above is coated on arigid silicon substrate, and reduce the coating film of the silvernanoparticles into a high conductive silver film by using thehigh-temperature calcination above 150° C. However, the process of usinghigh-temperature calcination also cannot be applied to thenon-heat-resistant flexible plastic substrate and it takes longerprocess time. And, adding the hydrogen in the high temperature toprocess reduction reaction will increase the safety risk.

It is therefore necessary to provide a method for forming a conductivefilm at room temperature to solve the problem of the conventionaltechnology.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a method forforming a conductive film at room temperature, which includes steps of:firstly adding AgNO₃ into a solution of dodecanoic acid; sequentiallydropping n-butylamine as a silver-ion ligand and a diluted aqueoussolution of a reducing agent to reduce silver ions into silvernanoparticles to initially obtain silver nanoparticles stably protectedby dodecanoate as a capping ligand; then using cyclohexane as along-term stabilizing solvent to spin-coat or print silver nanoparticlesusing dodecanoate as a capping ligand onto the surface of the substrateto form a patterned silver nanoparticle film; finally immersing thesubstrate into a high concentrated aqueous solution of a reducing agentto chemically reduce the patterned film of silver nanoparticles into aconductive silver film. This method can not only be used to convenientlyand rapidly print the patterned film or circuit on the substrate, butalso significantly increase the application potential of silvernanoparticles on the flexible substrate (i.e. PET substrate) which isnon-heat-resistant and low cost.

A secondary object of the present invention is to provide a method forforming a conductive film at room temperature, which includes steps of:firstly using cyclohexane containing 0.5 wt % dodecanoic acid as asolvent to mix with silver nanoparticles using dodecanoate as a cappingligand into a long-term stabilizing preservation solvent (30-daypreservation period) with 5.0 to 15.0 wt % silver nanoparticles, so thatsilver nanoparticles in the cyclohexane solution containing 0.5 wt %dodecanoic acid is unable to generate mutual aggregation andcondensation, and the size uniformity of the nanoparticles can be kept.Therefore, it can improve the printing quality of silver nanoparticlesapplied to inkjet printing process.

To achieve the above object, the present invention provides a method forforming a conductive film at room temperature, which comprises followingsteps of:

-   -   (a) adding AgNO₃ into a non-polar solvent containing dodecanoic        acid to be a first mixture;    -   (b) dropping n-butylamine into the first mixture as a ligand of        silver ions of silver nitrate to be a second mixture;    -   (c) dropping a diluted aqueous solution of a reducing agent into        the second mixture to be a third mixture, wherein the reducing        agent reduces silver ion into silver nanoparticles and        dodecanoate group of dodecanoic acid is used as a capping ligand        to be around and protect the silver nanoparticles;    -   (d) separating the dodecanoate-protected silver nanoparticles        from the third mixture;    -   (e) using cyclohexane as a solvent to mix with the        dodecanoate-protected silver nanoparticles to be a fourth        mixture;    -   (f) applying the fourth mixture onto a surface of a substrate to        form a film of the dodecanoate-protected silver nanoparticles;        and    -   (g) immersing the substrate into an aqueous solution of a        reducing agent to chemically reduce the film of silver        nanoparticles into a conductive silver film;    -   wherein the processes in the steps (a) to (g) are all performed        at room temperature.

In one embodiment of the present invention, in the step (a), the molarratio of the silver ions of AgNO₃ and the dodecanoate group ofdodecanoic acid is 1:2.

In one embodiment of the present invention, in the step (a), thenon-polar solvent is toluene.

In one embodiment of the present invention, in the step (b), the molarratio of the silver ions of AgNO₃ and n-butylamine is 1:2.

In one embodiment of the present invention, in the step (c), the dilutedaqueous solution of the reducing agent is selected from a dilutedaqueous solution of hydrazine, ascorbic acid, NaBH₄ or dimethylformamide(DMF).

In one embodiment of the present invention, in the step (c), the dilutedaqueous solution of the reducing agent is the diluted aqueous solutionof hydrazine, and the molar ratio of the silver ions of AgNO₃ andhydrazine of the diluted aqueous solution of hydrazine is 2:1.

In one embodiment of the present invention, in the step (c), the rangeof average particle diameter of the dodecanoate-protected silvernanoparticles is 6.20±0.57 nanometer (nm).

In one embodiment of the present invention, in the step (d), firstlyadding acetone into the third mixture to deposit thedodecanoate-protected silver nanoparticles, and then adopting methanoland acetone to wash, centrifuge and dry by reduced pressurecondensation, in order to obtain the dodecanoate-protected silvernanoparticles.

In one embodiment of the present invention, in the step (e), the fourthmixture is: firstly using cyclohexane containing 0.5 wt % dodecanoateacid as a solvent to mix with the silver nanoparticles in the step (d)into the fourth mixture which has 5.0 to 15.0 wt % silver nanoparticles,wherein the fourth mixture is a long-term stabilizing preservationsolvent (30-day preservation period) preferably having 10.0 wt % silvernanoparticles.

In one embodiment of the present invention, in the step (f), the fourthmixture is selected to be applied onto the surface of the substrate byspin-coating or inkjet printing.

In one embodiment of the present invention, in the step (f), thesubstrate is selected from the group consisting of a flexible plasticsubstrate, a glass substrate and a silicon wafer substrate; wherein theflexible plastic substrate is a polyethylene terephthalate (PET)substrate.

In one embodiment of the present invention, in the step (f), the aqueoussolution of the reducing agent is selected from an aqueous solution ofhydrazine, ascorbic acid, NaBH₄ or dimethylformamide (DMF).

In one embodiment of the present invention, in the step (f), the aqueoussolution of the reducing agent is an aqueous solution of hydrazine, andthe hydrazine concentration of the aqueous solution of hydrazine isbetween 70 and 90 wt %, such as 80 wt %.

In one embodiment of the present invention, the processes in the steps(a) to (g) are all performed at room temperature in the range between 20and 30° C., such as 25° C. In one embodiment of the present invention,after the step (g), the method further comprises: (g1) applying aheat-treatment to the conductive silver film at 100° C.

In one embodiment of the present invention, after the step (g), themethod further comprises: (h) repeating the steps (f) and (g) to furtherstack and form another conductive silver film on the original conductivesilver film.

In one embodiment of the present invention, after the step (h), themethod further comprises: (h1) applying a heat-treatment to two of thestacked conductive silver films at 100° C.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of steps (a) to (d) of a method forforming a conductive film at room temperature according to a firstembodiment of the present invention;

FIG. 2 is a schematic diagram of steps (e) to (g) of the method forforming the conductive film at room temperature according to the firstembodiment of the present invention; and

FIG. 3 is a schematic diagram of steps (e) to (g) of a method forforming a conductive film at room temperature according to a secondembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The structure and the technical means adopted by the present inventionto achieve the above and other objects can be best understood byreferring to the following detailed description of the preferredembodiments and the accompanying drawings. Furthermore, directionalterms described by the present invention, such as upper, lower, front,back, left, right, inner, outer, side, longitudinal/vertical,transverse/horizontal, and etc., are only directions by referring to theaccompanying drawings, and thus the used directional terms are used todescribe and understand the present invention, but the present inventionis not limited thereto.

Please refer to FIGS. 1 to 3, a method for forming a conductive film atroom temperature according to a first embodiment of the presentinvention mainly comprises the following steps: (a) adding AgNO₃ into anon-polar solvent containing dodecanoic acid (C₁₁H₂₃COOH, also known asn-dodecanoic acid) to be a first mixture; (b) dropping n-butylamine intothe first mixture as a ligand of silver ions of silver nitrate to be asecond mixture; (c) dropping a diluted aqueous solution of hydrazine(N₂H₄.H₂O) into the second mixture to be a third mixture, wherein thehydrazine reduces silver ion into silver nanoparticles (NPs) and thedodecanoate group of dodecanoic acid is used as a capping ligand to bearound and protect the silver nanoparticles; (d) separating thedodecanoate-protected silver nanoparticles (Ag—C₁₁H₂₃CO₂) from the thirdmixture; (e) using cyclohexane as a solvent to mix with thedodecanoate-protected silver nanoparticles to be a fourth mixture; (f)coating the fourth mixture onto a surface of a substrate by spin-coatingto form a film of dodecanoate-protected silver nanoparticles; and (g)immersing the substrate into an aqueous solution of hydrazine tochemically reduce the silver nanoparticles into a conductive silverfilm; wherein the processes in the steps (a) to (g) are all performed atroom temperature. FIGS. 1-3 of the present invention will be used todescribe the implementation details and reaction principles of each stepof the first and second embodiments.

Please refer to FIG. 1, the first step (a) of the method for forming aconductive film at room temperature according to the first embodiment ofthe present invention is to add AgNO₃ into a non-polar solventcontaining dodecanoic acid to be a first mixture. In the step, thepresent invention executes some sub-steps of: adopting 31.69 ml oftoluene as a non-polar solvent and adding 3.3387 g (16.67 mmol)Idodecanoic acid into toluene; stirring it until it is dissolved, thenadding 1.4156 g (8.33 mmol) of AgNO₃, which contains 0.25 mol/L Ag⁺,wherein AgNO₃ in the first mixture is used as a precursor of silverions, and in the following step (c), dodecanoic acid in the firstmixture will be used as a capping ligand of the silver nanoparticles.However, in the step, dodecanoic acid doesn't yet react with AgNO₃.Furthermore, in the step (a), the molar ratio of the silver ions ofAgNO₃ and the dodecanoate group of dodecanoic acid substantiallymaintains at 1:2. The non-polar solvent preferably is selected fromtoluene, but it can still be selected from other similar non-polarsolvents which have equivalent effect.

Please referring to FIG. 1, the following step (b) of the method forforming a conductive film at room temperature according to the firstembodiment of the present invention is to drop n-butylamine into thefirst mixture as a ligand of silver ions of AgNO₃ to be a secondmixture. In the step, the present invention executes the way of droppingper second to add the 1.6473 ml (16.67 mmol) of n-butylamine. Afterfinishing dropping within 2.5 minutes, the second mixture then continuesto react for 3.5 minutes and it gradually turned milky turbid suspensionin the process of n-butylamine is temporarily used as a ligand of silverions to separate silver ions from AgNO₃ firstly. And, the status of themilky turbid suspension shows that the second mixture includes thecomplex formed by coordinating silver ions and n-butylamine. In the step(b), the molar ratio of the silver ions of AgNO₃ and n-butylaminesubstantially maintains at 1:2.

Please refer to FIG. 1, the following step (c) of the method for forminga conductive film at room temperature according to the first embodimentof the present invention is to drop a diluted aqueous solution ofhydrazine (N₂H₄.H₂O) into the second mixture to be a third mixture. Inthe step, the present invention executes the way of firstly adding 80 wt% a solution of hydrazine (N₂H₄.H₂O) into 25 ml of a deionized water fordiluting and mixing to be a diluted aqueous solution of hydrazine as achemical redox fluid, which includes 0.2607 g (4.17 mmol) hydrazine.Then, until n-butylamine in the step (b) reacted for 3.5 minutes,executing the way of dropping per second to add the prepared dilutedaqueous solution of hydrazine into the second mixture to be a thirdmixture. The titration takes 15 minutes. After that, the third mixturereacts again for 3 hours, wherein hydrazine reduces silver ions usingn-butylamine as a temporary ligand into silver atoms in the metallicstate. Silver atoms are further re-aggregated to be silver nanoparticlesand the dodecanoate group of dodecanoic acid is used as a capping ligandto be around the silver nanoparticles. In the step (c), the molar ratioof the silver ions of AgNO₃ and hydrazine of the diluted aqueoussolution of hydrazine is 2:1.

In more detail, in step (c), silver ions using n-butylamine as atemporary ligand can be further reduced into silver atoms of themetallic state by hydrazine. And, a plurality of silver atoms of themetallic state is clustered with each other into nano-scale silvernanoparticles. After clustered, the average particle size of the silvernanoparticles is roughly in the range of 6.20±0.57 nm (nanometer),wherein each silver nanoparticles are actually formed by ten to hundredsof silver atoms clustered together. And, the outermost layer of thesilver nanoparticles reacts with dodecanoate to form ionic complex;wherein the outermost layer of the silver atoms will transform intopositively charged silver ions to bond with negative charge dodecanoategroups (as shown in the right-most side of FIG. 1). The dodecanoategroups are used as capping ligands to be around and protect the silvernanoparticles (Ag—C₁₁H₂₃CO₂) for avoiding the neighboring silvernanoparticles from clustering with each other again to enlarge theparticle size, and further it can effectively restrict the number ofsilver atoms clustered inside the silver nanoparticles is no longerincreased to stabilize the particle size of silver nanoparticles. Theadvantage of this is that the particle size of the silver nanoparticlescan be controlled stably. It can effectively avoid the problem that whenforming the patterned circuit in the following steps, the conductivequality of the circuit will be affected because of large conductiveparticles.

Please refer to FIG. 1, the following step (d) of the method for forminga conductive film at room temperature according to the first embodimentof the present invention is to separate the dodecanoate-protected silvernanoparticles (Ag—C₁₁H₂₃CO₂) from the third mixture. In this step, whenthe reaction of hydrazine is finished, adding 200 ml of acetone into thethird mixture to deposit the dodecanoate-protected silver nanoparticles,followed by using the solution of methanol and acetone to wash,centrifuge and dry by reduced pressure condensation, so that dark bluepowder of dodecanoate-protected silver nanoparticles can be obtained.However, the separation method of the present invention is not limitedthereto; the present invention can also use other existing separationtechniques or other solvents to separate the dodecanoate-protectedsilver nanoparticles from the third mixture.

Please refer to FIG. 2, the following step (e) of the method for forminga conductive film at room temperature according to the first embodimentof the present invention is to use cyclohexane as a solvent to mix withthe dodecanoate-protected silver nanoparticles to be a fourth mixture10. In the step, the present invention executes the way of firstly usingcyclohexane containing 0.5 wt % dodecanoate as a solvent and addingsilver nanoparticles of the step (d) into the solvent to mix to be afourth mixture 10 which has 5.0 to 15.0 wt % silver nanoparticles;wherein the fourth mixture 10 is a long-term stabilizing preservationsuspension, which preferably has 10.0 wt % silver nanoparticles. Becausethe fourth mixture 10 contains 0.5 wt % dodecanoate and cyclohexane, itis benefit to stably preserve silver nanoparticles using dodecanoate asthe capping ligand at least about 30 days or more. Thedodecanoate-protected silver nanoparticles can be stably suspended inthe cyclohexane solvent containing 0.5 wt % dodecanoate, and hence thefourth mixture 10 not only can be used for the storage backup purposes,but also can avoid the problem that silver nanoparticles further clusterto expand the particle diameter during the preservation.

Please refer to FIG. 2, the following step (f) of the method for forminga conductive film at room temperature according to the first embodimentof the present invention is to coat the fourth mixture 10 onto a surfaceof a substrate 20 by spin coating to form a film 11 ofdodecanoate-protected silver nanoparticles. In the embodiment, thefourth mixture 10 is selected to be applied onto the upper surface ofthe substrate 20 by spin coating via a spin-coating liquid distributor30; wherein the substrate 20 is selected from the group consisting of aflexible plastic substrate, a glass substrate and a silicon wafersubstrate and preferably selected from a flexible plastic substrate, forexample, the substrate made of polyethylene terephthalate (PET) ofnon-heat-resistant and low cost. In this step, the present inventionadopts the fourth mixture 10 containing 10 wt % dodecanoate-protectedsilver nanoparticles (Ag—C₁₁H₂₃CO₂) to execute spin coating wherein thesubstrate 20 is placed on the a turntable, and spin-coated for 15seconds at 2000 round per minute, so that the mixture 10 can beuniformly coated on the substrate 20 (e.g., PET substrate). Until thesubstrate 20 is dried by nitrogen, so that cyclohexane in the fourthmixture is completely volatilized. The surface of the substrate 20 canbe formed a film 11 of dodecanoate-protected silver nanoparticles.

Please refer to FIG. 2, the following step (g) of the method for forminga conductive film at room temperature according to the first embodimentof the present invention is to immerse the substrate 20 into an aqueoussolution of hydrazine to chemically reduce the film 11 of silvernanoparticles into a conductive silver film 12. In the step, theconcentration of hydrazine of the aqueous solution of hydrazine isobviously higher than that used in the step (c); wherein theconcentration of hydrazine of the aqueous solution of hydrazine in thestep is between 70 and 90 wt %, for example, preferably 80 wt %. In theembodiment, the present invention immerses the coated film 11 of silvernanoparticles of the step (f) into the aqueous solution of hydrazine(N₂H₄) at room temperature of 25° C. for one hour, and then chemicallyreduce it to obtain a conductive silver film 12. Finally, deionizedwater is used to wash the surface of the conductive silver film 12 andthe surface is blown dried by nitrogen gas (or by wind). In more detail,the hydrazine can reduce silver ions on the surface of the silvernanoparticles in the film 11 of the silver nanoparticles to the silveratoms in the metal state, and desorb dodecanoate from silvernanoparticles. After reducing and desorption, silver nanoparticles onlycontain silver atoms in the metal state, and closely attach and combineto the upper surface of the substrate 20.

It is worthy to note that the processes in the steps (a) to (g) are allperformed at room temperature. The room temperature of the presentinvention refers to the range between 0 and 50° C., and preferablybetween 10 and 40° C. and especially between 20 and 30° C., such as 21,23, 25, 27 or 29° C. Furthermore, the conductive silver film 12 formedby the steps (a) to (g) are specially suitable to be applied to theintegrated circuit (IC) on a wafer, a circuit design of a transparentconductive layer of a thin film transistor liquid crystal display(TFT-LCD) or repair of fine-pitch tiny opening defects.

Please refer to FIG. 3, the method for forming a conductive film at roomtemperature according to the second embodiment of the present inventionis illustrated and similar to the first embodiment, so that the secondembodiment uses similar terms or numerals of the first embodiment.However, compared with the first embodiment, the second embodimentcomprises the following steps: (a) adding AgNO₃ into a non-polar solventcontaining dodecanoic acid (C₁₁H₂₃COOH) to be a first mixture; (b)dropping n-butylamine into the first mixture as a ligand of silver ionsof silver nitrate to be a second mixture; (c) dropping a diluted aqueoussolution of hydrazine (N₂H₄.H₂O) into the second mixture to be a thirdmixture, wherein the hydrazine reduces silver ion into silvernanoparticles and uses the dodecanoate group of dodecanoic acid as acapping ligand to be around and protect the silver nanoparticles; (d)separating the dodecanoate-protected silver nanoparticles (Ag—C₁₁H₂₃CO₂)from the third mixture; (e) using cyclohexane as a solvent to mix withthe dodecanoate-protected silver nanoparticles to be a fourth mixture;(f) inkjet printing the fourth mixture onto a surface of a substrate toform a film of dodecanoate-protected silver nanoparticles; and (g)immersing the substrate into an aqueous solution of hydrazine tochemically reduce the silver nanoparticles into a conductive silverfilm; wherein the processes in the steps (a) to (g) are all performed atroom temperature.

In the second embodiment of the present invention, the difference isthat: as shown in FIG. 3, the step (f) of the second embodiment furtheruses the way of inkjet printing to replace that of spin-coating. In thesecond embodiment of the present invention, the fourth mixture 10 isselected to be applied onto the upper surface of the substrate 20 byinkjet printing via an inkjet liquid distributor 40; wherein thesubstrate 20 is selected from the group consisting of a flexible plasticsubstrate, a glass substrate and a silicon wafer substrate andpreferably selected from a flexible plastic substrate, for example, thesubstrate formed by polyethylene terephthalate (PET) ofnon-heat-resistant and low cost.

In this step (f), the present invention adopts the fourth mixture 10containing 10 wt % dodecanoate-protected silver nanoparticles(Ag—C₁₁H₂₃CO₂) to inkjet printing; wherein the substrate 20 is placed onthe a work platform or a mobile platform, and then the upper surface ofthe substrate 20 is inkjet-printed by the inkjet liquid distributor 40,so that the mixture 10 can be uniformly inkjet-printed on the substrate20 (e.g., PET substrate). Until the substrate 20 is dried by nitrogen(or by wind) so that cyclohexane in the fourth mixture 10 is completelyvolatilized, the surface of the substrate 20 can be formed a film 13 ofdodecanoate-protected silver nanoparticles, which can be the patternedshape of circuits. In the following step (g), immerse the substrate 20into the aqueous solution of hydrazine to reduce the film 13 of silvernanoparticles to the conductive silver film 14. The steps (a) to (e) and(g) of the second embodiment are almost similar to the first embodiment,and therefore the description thereof is no longer to be repeatedtherein.

On the other hand, according to other embodiments of the presentinvention, in the step (c) of the first and second embodiments describedabove, the diluted aqueous solution of hydrazine can be replaced withthe diluted aqueous solution of other reducing agents, such as dilutedaqueous solution of ascorbic acid, NaBH₄ or dimethylformamide (DMF).Meanwhile, the aqueous solution of the hydrazine in the step (g) can bereplaced with the aqueous solution of other reducing agents, such asaqueous solution of ascorbic acid, NaBH₄ or dimethylformamide (DMF). Theabove agents are formulated into different concentrations of thereducing agents. The capabilities of electronic conduction of the silverconductive film are obtained by using various reducing agents, as shownin Table 1:

TABLE 1 Thickness Reductant Reaction time Resistivity (μΩcm) (nm) 80 wt% Hydrazine 1 hr 4.80-9.90  60-250 1M ascorbic acid 1 hr 13.4-17.2110-121 0.005M NaBH₄ 1 min  91.2-112.9 175-240 50% DMF 2 hr 249.1-377.0145-180

In addition, after the step (g), the method can optionally furtherinclude a sub-step (g1): applying a heat-treatment to the conductivesilver film at 100° C. for 1 hour to density the silver conductive filmto enhance the conductivity of the film. For example, the resistivity(μΩcm) can be reduced from 4.80-9.90 μΩcm to 2.05-4.75 μΩcm.

Furthermore, after the step (g) or (g1), the method can optionallyfurther include a step (h): repeating steps (f) and (g) to further stackand form another conductive silver film on the original conductivesilver film to enhance the conductivity of the film by increasing thethickness and filling the holes of the first layer of conductive film.And, the results are shown in Table 2 as follows:

TABLE 2 Reductant Reaction Resistivity of two layer Thickness of two (Inthe step time in of conductivity Ag film layer of conductivity h) thestep h (μΩcm) Ag film (nm) 80 wt % 1 hr 3.23-4.62 183-349 Hydrazine 1Mascorbic 1 hr 4.34-5.95 203-278 acid 0.005M 1 min 6.50-10.9 181-256NaBH₄ 50% DMF 2 hr 52.3-78.3 165-237

Similarly, after the step (h), the method can optionally further includea sub-step (h1): applying a heat-treatment to the conductive silver filmat 100° C. for 1 hour to density the silver conductive film to enhancethe conductivity of the film.

As described above, compared with the conditional method of forming aconductive film using decanoate-protected silver nanoparticles at roomtemperature, the stability of nanoparticles is poor, and hightemperature calcination over 150° C. is adopted to reduce the coatedfilm of silver nanoparticles on the rigid silicon substrate to highconductive silver film, and it is not suitable to the flexible plasticsubstrate of non-heat-resistance and has the shortcomings of the processtime-consuming and high safety risk. FIG. 1-3 according the presentinvention shows that firstly adding AgNO₃ into a solution of dodecanoicacid; sequentially dropping n-butylamine as a silver-ion ligand and adiluted aqueous solution of a reducing agent to reduce silver ions intosilver nanoparticles to initially obtain dodecanoate-protected silvernanoparticles stably; then using cyclohexane as a long-term stabilizingsolvent to spin-coat or print the dodecanoate-protected silvernanoparticles on the surface of the substrate to form a patterned silvernanoparticle film; finally immersing the substrate into a highconcentrated aqueous solution of a reducing agent to chemically reducethe patterned film of silver nanoparticles into a conductive silverfilm. This method can not only be used to conveniently and rapidly printthe patterned film or circuit on the substrate, but also significantlyincrease the application potential of silver nanoparticles on theflexible substrate (ie. PET substrate) which is non-heat-resistant andlow cost.

Furthermore, the present invention includes steps of: firstly usingcyclohexane containing 0.5 wt % dodecanoic acid as a solvent to mixdodecanoate-protected silver nanoparticles into a long-term stabilizingpreservation solvent (30-day preservation period) with 5.0 to 15.0 wt %silver nanoparticles, so that silver nanoparticles in the cyclohexanesolution containing 0.5 wt % dodecanoic acid is unable to generatemutual aggregation and condensation, and the size uniformity of thenanoparticles can be kept. Therefore, it can improve the printingquality of silver nanoparticles applied to inkjet printing process.

The present invention has been described with a preferred embodimentthereof and it is understood that many changes and modifications to thedescribed embodiment can be carried out without departing from the scopeand the spirit of the invention that is intended to be limited only bythe appended claims.

What is claimed is:
 1. A method for forming a conductive film at roomtemperature, comprising: (a) adding AgNO₃ into a non-polar solventcontaining dodecanoic acid to be a first mixture; (b) droppingn-butylamine into the first mixture as a ligand of silver ions of silvernitrate to be a second mixture; (c) dropping a diluted aqueous solutionof a reducing agent into the second mixture to be a third mixture,wherein the reducing agent reduces silver ion into silver nanoparticlesand dodecanoate group of dodecanoic acid is used as a capping ligand tobe around and protect the silver nanoparticles; (d) separating thesilver nanoparticles using dodecanoate as the capping ligand from thethird mixture; (e) using cyclohexane as a solvent to mix with silvernanoparticles using dodecanoate as the capping ligand to be a fourthmixture; (f) applying the fourth mixture onto a surface of a substrateto form a film of silver nanoparticles using dodecanoate as the cappingligand; and (g) immersing the substrate into an aqueous solution of areducing agent to chemically reduce the film of silver nanoparticlesinto a conductive silver film; wherein the processes in the steps (a) to(g) are all performed at room temperature.
 2. The method for forming aconductive film at room temperature according claim 1, wherein in thestep (a), the molar ratio of the silver ions of AgNO₃ and thedodecanoate group of dodecanoic acid is 1:2.
 3. The method for forming aconductive film at room temperature according claim 1, wherein in thestep (a), the non-polar solvent is toluene.
 4. The method for forming aconductive film at room temperature according claim 1, wherein in thestep (b), the molar ratio of the silver ions of AgNO₃ and n-butylamineis 1:2.
 5. The method for forming a conductive film at room temperatureaccording to claim 1, wherein in the step (c), the diluted aqueoussolution of the reducing agent is selected from a diluted aqueoussolution of hydrazine, ascorbic acid, NaBH₄ or dimethylformamide (DMF).6. The method for forming a conductive film at room temperatureaccording to claim 5, wherein in the step (c), the diluted aqueoussolution of the reducing agent is the diluted aqueous solution ofhydrazine, and the molar ratio of the silver ions of AgNO₃ and hydrazineof the diluted aqueous solution of hydrazine is 2:1.
 7. The method forforming a conductive film at room temperature according ti claim 1,wherein in the step (c), the range of average particle diameter of thedodecanoate-protected silver nanoparticles is 6.20±0.57 nm.
 8. Themethod for forming a conductive film at room temperature according toclaim 1, wherein in the step (d), firstly adding acetone into the thirdmixture to deposit the dodecanoate-protected silver nanoparticles as thecapping ligand, and then adopting methanol and acetone to wash,centrifuge and dry by reduced pressure condensation, in order to obtainthe dodecanoate-protected silver nanoparticles.
 9. The method forforming a conductive film at room temperature according to claim 1,wherein in the step (e), the fourth mixture is treated by firstly usingcyclohexane containing 0.5 wt % dodecanoate acid as a solvent to mixsilver nanoparticles in the step (d) into the fourth mixture which has5.0 to 15.0 wt % silver nanoparticles.
 10. The method for forming aconductive film at room temperature according to claim 1, wherein in thestep (f), the fourth mixture is selected to apply onto the surface ofthe substrate by spin-coating or inkjet printing.
 11. The method forforming a conductive film at room temperature according to claim 1,wherein in the step (f), the substrate is selected from the groupconsisting of a flexible plastic substrate, a glass substrate and asilicon wafer substrate.
 12. The method for forming a conductive film atroom temperature according to claim 11, wherein the flexible plasticsubstrate is a polyethylene terephthalate substrate.
 13. The method forforming a conductive film at room temperature according to claim 1,wherein in the step (f), the aqueous solution of the reducing agent isselected from an aqueous solution of hydrazine, ascorbic acid, NaBH₄ ordimethylformamide (DMF).
 14. The method for forming a conductive film atroom temperature according to claim 13, wherein in the step (f), theaqueous solution of the reducing agent is an aqueous solution ofhydrazine, and the hydrazine concentration of the aqueous solution ofhydrazine is between 70 and 90 wt %.
 15. The method for forming aconductive film at room temperature according to claim 1, wherein theprocesses in the steps (a) to (g) are all performed at room temperaturein the range between 20 and 30° C.
 16. The method for forming aconductive film at room temperature according to claim 1, wherein afterthe step (g), the method further comprises: (g1) applying aheat-treatment to the conductive silver film at 100° C.
 17. The methodfor forming a conductive film at room temperature according to claim 1,wherein after the step (g), the method further comprises: (h) repeatingthe steps (f) and (g) to further stack and form another conductivesilver film on the original conductive silver film.
 18. The method forforming a conductive film at room temperature according to claim 17,wherein after the step (h), the method further comprises: (h1) applyinga heat-treatment to two of the stacked conductive silver film at 100° C.